E-mail send and recieve problem

I have installed ISPConfig on my server and add a website to it. Also have I add a e-mail address to that website. Now I have try to send a e-mail from my e-mail address (of my internet provider) to my new e-mail address from the website. Then I get the following error:

Recipient address rejected: Relay access denied

I have read the ather posts about this topic but they don't answer my question.

The option mail dir in management>server>settings was not checked. I have now checked it. But I have probblems loging in to the web interface to send a test mail. How do I log in here, by that I mean which username and password must I use. Must I type in by e-mail the e-mail address of the user of the username?

Is the recipient address domain in /etc/postfix/local-host-names?
Does your mail actually arrive on your ISPConfig machine, or is it sent to another system because of some DNS misconfiguration? Have a look into your maillog (under /var/log) to see if the mail is at least arriving on your system.

You can run

Code:

tail -f /var/log/[your_maillog_file]

to watch real-time what happens when you try to send a mail to your ISPConfig system.

Looks ok. So I guess that emails are arriving but that you have a problem with Maildir vs. mbox. For example, if you have checked Maildir under Management -> Server -> Settings -> Email, but your POP3 daemon uses mbox then your POP3 daemon cannot access your mail because they're in Maildir (and vice versa).
So if you've checked Maildir in ISPConfig, uncheck it; if you haven't checked it, then check it now.